It is known to dispense with the need for a considerable portion of the coke of the charge of a blast furnace by the replacement of the hot air blasted into the main tuyeres by a certain amount of hot reducing gases. These reducing gases are injected at a high temperature and contain chiefly CO and H.sub.2.
A method of this type is in particular advocated in order to reduce coke consumption, coke being a fuel which is costly and in relatively short supply. It is even possible to envisage all the heat and reducing gas requirements being covered by this method. In this case, in addition to its metal carburizing function, the coke would then only have the role of a refractory mechanical support which supports the solid charge and enables the decanting of the reduced metal and the slag.
There are at present several methods of manufacturing reducing gases, for example the method using partial oxidation under steam or the method using partial oxidation with oxygen, applied to gaseous or liquid hydrocarbons.
In an attempt to diversify the energy sources used in the iron and steel industry, the applicants have also previously advocated the production of reducing gases from a gaseous or liquid hydrocarbon or from a carbonaceous solid material, suh as coal, using the throat gas as the oxidising agent.
In all cases, the reducing gases obtained are heated to a temperature of between 1700.degree. C. and 2500.degree. C. in order to be injected into the hearth of the blast furnace. High temperatures may be obtained in several ways, but excellent results have been obtained according to a method advocated elsewhere by the applicants by heating these gases in a plasma medium (GB patent specification No. 1 488 976). Within the scope of this method, the applicants suggested in particular to provide the inlet pipe adjacent to the main tuyere with a plasma oven, preferably cylindrical, whose gas outlet axis was oblique to the longitudinal axis of the tuyere. An arrangement of this type raises, however, the problem of the suitable orientation of the gas outlet axes of the inlet pipe and the plasma oven to ensure that the resultant mixture is directed along the longitudinal axis of the tuyere and thus to prevent the gases from striking the internal refractory covering of the tuyere.